<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>p1</title>
    
    <style>
        div{
            width: 100px;
            height: 100px;
            background-color: #f00;
        }

        .d2:hover{
            background-color: #000;
        }
        .d3{
            font-size: 50px;
            border: 5px solid blue;
            border-radius: 15px;
            box-shadow: 5px 5px 5px 5px #000;
            background-image: linear-gradient(purple,blue,pink);
        }

        .h3{
            background-color: yellow;
        }
    </style>
    
</head>
<body>

<div class="d1" style="color: blue">1</div>
<div class="d2">2</div>
<div class="d3">3</div>

<hr>

<h3 class="h3">标题1</h3>
<h3 class="h3">标题2</h3>
<h3>标题3</h3>
<h3>标题4</h3>

</body>
</html>